The invention relates to the field of transmitting digital data by optical means. It is more particularly concerned with transmission at high bit rates on long-haul fiber optic links.
More detailed the invention is related to a frequency adjustment scheme for an optical filter.
The invention is also related to a transmission system with a transmitter function, a transmitting fiber and a receiver function and the transmitter function comprises light sources, with modulators and a wavelength multiplexer and the receiver function comprises, a wavelength demultiplexer, adjustable filters and receivers.
Such transmission scheme uses an optical transmitter connected to an optical receiver by the fiber link. The transmitter generally modulates the power of an optical carrier wave from a laser oscillator as a function of the information to be transmitted. NRZ or RZ modulation is very frequently used and entails varying the power of the carrier wave between two levels: a low level corresponding to extinction of the wave and a high level corresponding to a maximum optical power. The variations of level are triggered at times imposed by a clock rate and this defines successive time cells allocated to the binary data to be transmitted. By convention, the low and high levels respectively represent the binary values “0” and “1”.
The maximum transmission distance is generally limited by the ability of receivers to detect without error these two power levels after the modulated wave has propagated in the optical link. The usual way to increase this distance is to increase the ratio between the average optical power of the high levels and that of the low levels; this ratio defining the “extinction ratio” which is one of the characteristics of the modulation.
For a given distance and a given extinction ratio, the information bit rate is limited by chromatic dispersion generated in the fibers. This dispersion results from the effective index of the fiber depending on the wavelength of the wave transported, and it has the consequence that the width of the transmitted pulses increases as they propagate along the fiber.
This phenomenon is characterized by the dispersion coefficient D of the fiber, which is defined as a function of the propagation constant β by the equation D=−(2πc/λ2)d2β/dω2, where λ and ω are respectively the wavelength and the angular frequency of the wave.
Not only chromatic dispersion limits the possibility of transmission, but it is a main factor for distortion. Increasing the data rate up to higher levels—we are talking about T Bit/s—the effects of the fibers increase the impact on the received signal. One solution is the use of DWDM (dense wavelength division multiplex) systems to increase the bit rate. The wavelength channels are selected in a way that the information of the single channels can be selected at receiver side and analyzed with an acceptable bit/error rate.
Again the bit rate is limited by the spectrum of the channels.
A modulation scheme know as VSB (vestigial side bond modulation) is explained in “5.12 Tbit/s Transmission over 3×100 km of Teralight fiber” Bigo, S. et al., paperPD2, PP40-41, ECOC 2000.
The two side bands of a NRZ spectrum generally contain redundant information. It is therefore tempting to filter out one of them in order to increase spectral efficiency, a technique known as VSB. However VSB is difficult to implement at the transmitter because the suppressed side bands rapidly reconstruct through fiber non-linearities.
So a VSB filtering at the receiver side is proposed. With modulation and filtering scheme like VSB the bandwidth efficiency increase to a value of more than 0.6 bit/s/Hz compared with 0.4 bit/s/Hz in conventional systems.
Again the transmission is limited due to the effects of cross talking between the adjacent channels.
One critical point is the fine adjustment of the optical filter either at the transmitter or the receiver side of the transmission line. The positioning of the filter in the wavelength multiplex limits the maximum useable bandwidth of the transmission system. One conventional approach to lock the wavelength division multiplex filter onto a given channel wavelength is to maximize in a feed back loop the output power, so that the maximum of the transmission is found. For VSB modulation schemes the filter must be locked on one of the edges of the transmission line and therefore this method cannot apply to VSB transmission lines.
In the EP 1143 643 a filter is used to filter optical sidebands by a measurement of both reflecting and transmitting part of the filter and a comparison between the two derived signals. When both levels counts equal the filter is well positioned with this method.
This filter doesn't run in a VSB transmission scheme with alternate channel spaces as described in the application EP 2001440033. Furthermore the method of EP 1143643 can be hardly used for a proper filter positioning. The condition of equality between transmitted and reflected powers does not correspond to optimal performance of the system. Indeed it has been demonstrated experimentally that a VSB filter must exhibit a particular asymmetric intensity phase shape. The suppression of side band through the narrow filter must be thorough in order to obtain good bit-error performance.